This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. This study aims to determine the structure of the membrane ion channel, KcsA, in open and closed states. Terbium labeled KcsA mutants are incorporated into unilamellar lipid vesicles and exposed to X-rays at energies near the Tb+3 L3 edge. This solution X-ray scattering experiment takes advantage of an anomalous signal arising from cite specific Tb+3 labels.